Solenoid valve

ABSTRACT

The invention relates to a solenoid valve made using certain materials which is capable of operation at high frequencies and which can be made as a compact unit. The plunger ( 1 ) is made from an electromagnetic material having a saturation flux density greater than 1.2 Tesla preferably more than 1.6 Tesla, the bore leading from the valve head chamber ( 14 ) to the nozzle orifice ( 12 ) has a length to diameter ratio of less than 5:1 preferably between 2:1 and 4:1 and the nozzle orifice ( 12 ) has a diameter of 80 micrometer or less. The invention also relates to method for operating a drop on demand ink jet printer incorporating such a valve.

RELATED APPLICATIONS

The present application is a division of U.S. application Ser. No.10/492,258, filed Dec. 28, 2004, which is the United States nationalstage application of PCT/CA/01544, filed Oct. 15, 2002, which claimspriority to British application 0216935.7, filed Jul. 22, 2002, all ofwhich are hereby incorporated by reference in its entirety.

The present invention relates to a device, notably a high speed solenoidtype valve, an ink jet printer incorporating that valve and methods ofoperating an ink jet printer, preferably utilising that valve.

BACKGROUND OF THE INVENTION

Ink jet printers are non-contact printers in which droplets of ink areejected from one or more nozzle orifices so as progressively to build upa printed image on a substrate moved relative to the nozzle. One form ofink jet printer comprises a source of ink, typically a reservoir orbottle of ink, which is pressurised to from 0.1 to 2 bar, notably about1 bar. The pressure is created, for example, by pressurising the airspace above the ink in the bottle or reservoir. The ink is fed to thenozzle orifice(s) in a print head through which it is ejected as aseries of droplets onto the surface of the substrate. The flow of inkthrough each nozzle orifice is controlled by a solenoid valve.Typically, such a valve comprises an electromagnetic plunger journalledfor axial movement within an axially extending electric coil. The distalend of the plunger is located within a valve head chamber through whichink flows from the reservoir to the nozzle orifice. When current is fedthrough the coil, this generates a magnetic field which acts on theplunger to move it axially and thus open, or shut, the inlet of a borefrom the valve head chamber to the nozzle orifice. Typically, themagnetic field acts to retract the plunger against the bias of a coilspring to create a flow path between the valve head chamber and thenozzle orifice. When the electric current no longer flows in the coil,the magnetic field ceases and the plunger returns under the bias of thespring to close the flow path to the nozzle orifice. Typically, aplurality of nozzle orifices are formed as one or more rows in a plate,the nozzle plate, and each nozzle orifice is served by a separatesolenoid valve, so that droplets of ink can be ejected independentlyfrom one or more of the nozzle orifices. Typically, the valves are fedwith ink from the reservoir via a manifold which serves to split andeven the ink flow between each of the valves. The row of nozzle orificesis typically aligned transversely to the direction of travel of thesubstrate so that simultaneous operation of the valves will cause a rowof ink dots to be printed on the substrate.

The valves are operated so as to deposit dots upon the substrate at thedesired locations on the substrate to build up the elements of a five,seven, eight or more dot raster image on the substrate. By suitabletiming of the opening of the various valves in the print head analphanumeric or other image can be formed on the substrate to print adate, product batch code, logo, bar code or other image on thesubstrate. If desired, several print heads can be combined in an arrayso as to print a wider image on the substrate and the line of nozzleorifices in the print head can be angled to the direction of movement ofthe substrate so as to lay down droplets which are more closely spacedthan where the print head is aligned normal to the line of travel of thesubstrate.

For convenience, the term drop on demand printer will be used to denotein general such types of ink jet printer.

The size of the printed dot can readily be altered by varying theduration for which the valve is held open, and hence the amount of inkthat it allows to flow through the nozzle orifice. The form of the imagewhich is printed can readily be altered by varying the sequence ofoperation of the valves in the print head so that droplets are ejectedfrom the appropriate nozzles in the appropriate sequence to form thedesired image. Such alterations of the images and the dot sizes canreadily be controlled by a computer or microprocessor operating under anappropriate program or operating system. Such drop on demand printersare widely available commercially and find widespread use in printing awide range of both visible and non-visible machine-readable images on awide range of substrates.

However, as the speed of travel of the substrate past the print headincreases, a point is reached at which the valve cannot be operated atsufficient speed to eject droplets at sufficient frequency to form thedesired image without creating some distortion. Typically, the limit forthe speed of operation of solenoid valves in current use in an ink jetprinter head is less that 800 to 1000 Hz. With increasing pressure onmanufacturers to increase through put from a given production orpackaging line, there is an increasing need to be able to print the dotsonto the substrate at rates greater than this.

In an alternative form of ink jet printer known as an impulse jetprinter, a piezoelectric crystal or other transducer is applied to orforms part of a wall of an ink jet chamber having an ink inlet and anink outlet to a nozzle orifice. When a voltage is applied to thetransducer, the transducer expands or flexes and causes a change in thevolume of the ink jet chamber. This causes a droplet of ink to beejected from the chamber and to exit through the nozzle orifice. Thetransducer can be caused to flex at very high rates by electroniccontrol of the frequency of the electrical pulses applied to thetransducer, so that such a print head can apply dots at frequencies upto 15 kHz or more. However, the volume of ink ejected through the nozzleorifice is dependent upon the extent of flexing of the transducer. Thiscan be varied by varying the amplitude of the electric pulse applied tothe transducer. However, each type of transducer operates consistentlyonly within a narrow percentage, typically plus or minus 50%, of theoptimum operating pulse amplitude, so that only a limited range of dotsizes can be achieved with a commercially available impulse jet printer.This limits the number of applications for which a given impulse jethead can be used for.

It has been proposed in International Patent Application NoPCT/SE97/01007 to produce a solenoid type valve for a drop on demand inkjet printer which is claimed to be capable of operating at frequenciesof up to 3 kHz. Such a valve incorporates light weight components so asto reduce the inertia of the plunger and thus enable it to accelerateand decelerate rapidly at each extreme of its travel within the coil. Toachieve this, the plunger is formed from two components, one made froman electromagnetic material so that it can be caused to move by themagnetic field generated by the current passing through the coil, and asecond lightweight plastic component for the distal end of the plunger.Such a construction is complex and expensive. Furthermore, we have foundthat a print head incorporating such a valve design does not printacceptable images. For example, at high frequencies of operation of thevalve, the printed dots are uneven and there are many small satellitedots around each of the primary dots printed by the print head.

We have now devised a form of valve which can be operated at speeds ofup to 8 kHz or more and yet can be used in a drop on demand printer toprint uniformly sized droplets over a surprisingly wide range of dotsizes and operating frequencies. Furthermore, the valve of the inventioncan be more compact and with smaller components than a conventionaldesign of solenoid valve for use in a drop on demand ink jet printer.This allows high definition printing to be achieved at high print rateswithout excessive heat being generated during operation of the valve.Such a valve enables drop on demand technology to be used in high speedapplications for which an impulse jet print head had hitherto beenconsidered the only technically viable form of print head.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides a valve mechanism forcontrolling the flow of fluid therethrough, which mechanism comprises aplunger member journalled for axial reciprocation between a rest and anoperative position within a tubular member supporting an electric coilunder the influence of a magnetic field generated by that coil when anelectric current passes through the coil, bias means to bias the plungertowards its rest position when no current is applied to the coil, thedistal end of the plunger extending into a valve head chamber having aoutlet bore to a nozzle orifice, the reciprocation of the plunger beingadapted to open or close a fluid flow path from the valve head chamberthrough that bore to the nozzle orifice, characterised in that:

a. the plunger is made from an electromagnetic material having asaturation flux density greater than 1.2 Tesla; and

b. the bore leading from the valve head chamber to the nozzle orificehas a length to diameter ratio of 5:1 or less; and

c. the nozzle orifice has a diameter of 80 micrometres or less.

For convenience, the terms distal and proximal will be used herein todenote that portion of a component which is located downstream andupstream respectively with respect to the flow of ink or other fluidthrough the valve. The valve of the invention is of especial applicationin drop on demand ink jet printers which are to be operated at dropgeneration frequencies of 1 kHz and higher. For convenience, theinvention will be described hereinafter in terms of a valve for such anapplication.

In existing designs of solenoid valve, it is not possible to operate thevalve for prolonged periods at speeds in excess of 1 kHz. We believethat this is due to the fact that the hysteresis of the material fromwhich the plunger is made to changes in the magnetic field applied to itcauses the material to become magnetically saturated so that it nolonger can respond to further changes in the magnetic field applied toit. The use of a material having a magnetic saturation flux density ofgreater than 1.2 Tesla, preferably from greater than 1.4 Tesla, notably1.4 to 1.8 Tesla, enables the plunger to continue to respond to changesin the magnetic field applied to it even after prolonged periods ofoperation at frequencies in excess of 3 kHz and more, for example up to9 kHz, without becoming magnetically saturated. Furthermore, where thecoil/plunger combination also has a high magnetic inductance, forexample 9 milliHenrys or more, the coil can exert a high driving forceon the plunger further enhancing the rate of response of the plunger tochanges in the electrical current flowing through the coil. Theseproperties enable a smaller plunger, that is one of smaller mass, to beused, since a given drive current can exert a greater force on theplunger of a given size than where a conventional material ofconstruction is used. These properties result in a plunger which can beaccelerated rapidly at either extreme of its travel with a comparativelylow driving current, thus reducing the heat generated during operationof the valve.

We have also found that it is desirable to reduce the gap between thecoil and the plunger to a minimum so as to reduce the loss of magneticcoupling between the coil and the plunger to a minimum. This enhancesthe response of the plunger to changes in the current applied to thecoil and reduces the amplitude of the current required to drive theplunger within the coil. In conventional designs of solenoid valveproposed hitherto, the coil is wound upon a plastic or other bobbin andthe bobbin then mounted upon a tube forming the bore within which theplunger reciprocates. This results in the formation of a radial gap of 2mms or more between the inner face of the coil and the outer face of theplunger. From another aspect of the present invention, it is preferredto form the coil directly upon or within the wall of the tubular memberwithin which the plunger reciprocates as described below, so as toreduce this radial gap to less than 1 mm.

The nozzle orifice is typically provided by the open end of the outletbore from the valve head chamber. Such a bore can be provided by acapillary tube outlet to the valve head chamber, or by the bore in ajewel nozzle of the type conventionally used in ink jet printer nozzles.Such a jewel can be set into the distal end of a bore in a nozzle plateupon which the valve mechanism is mounted. It will be appreciated thatthe jewel nozzle can provide both the outlet bore from the valve headchamber and the nozzle orifice itself, for example when a jewel nozzleis set into a bore within a nozzle plate so that its proximal face issubstantially flush with the proximal face of the nozzle plate, forexample where it forms all or part of the distal end wall of the valvehead chamber.

We have found that if the volume of the body of fluid retained in thebore between the valve head chamber and the nozzle orifice is excessive,that body of fluid possesses sufficient inertia to damp out rapidmovement of the plunger required when the valve is operated at highfrequencies. This causes the fluid to issue erratically from the nozzleorifice at frequencies of operation in excess of about 1 kHz.Surprisingly, if the length of the bore is less than 5 times thediameter d of the bore, for example 0.5 to 5 times the diameter of thebore, the volume of ink retained in the bore between actuations of theplunger is reduced sufficiently so that it can respond rapidly toactuations of the plunger.

Where the plunger is to be actuated at frequencies in excess of 1 kHz,we have found that if the length to diameter ratio of the bore isreduced to below about 1.5:1, spraying of the droplets at the nozzleorifice may occur, even though the frequency of droplet formation isbelow than at which an impulse jet printer having an even smaller lengthto diameter ratio for the outlet nozzle operates without causingspraying of the ejected droplets. We therefore prefer that the length todiameter ratio of the bore be from 2:1 to 4:1. It is particularlypreferred that the length to diameter ratio be less than about 3:1,preferably 2:1 to 3:1. To reduce the volume of ink within the bore, wehave found that the diameter of the nozzle orifice should be less than80 micrometres, preferably about 40 to 60 micrometres. Surprisingly, wehave found that the use of these combinations of bore length to diameterand nozzle orifice sizes reduces the formation of satellite dropletsfrom the nozzle as well as reducing the damping effect on the motion ofthe plunger and enables droplets of a wide range of sizes to be producedconsistently at frequencies in excess of 1 kHz, notably at 2 kHz ormore.

With conventional valve designs, one problem has been the imperfectformation of the desired droplets after the valve has been idle for anylength of time. This has been ascribed to drying out of the ink at thenozzle orifice and many steps have been taken to reduce this. We havefound that the use of the bore and nozzle orifice dimensions describedabove has the surprising effect of reducing the imperfect initialdroplet formation. We believe that this is due to a reduction in thedrying out of ink within the valve mechanism. Furthermore, we have foundthat the use of a length to diameter ratio of from 2:1 to 5:1 enablescontrol of the directionality of the ejection of the droplet from thenozzle orifice to be achieved. In a conventional design of drop ondemand print head, the length to diameter ratio of the bore is typicallyabout 10:1. It is surprising that reducing the length of the bore doesnot result in loss of directionality of the ejection of the droplet fromthe nozzle orifice.

Surprisingly, we have found that the removal of part of the core of theplunger to form an internal bore within part of the length of theplunger and thus reduce the mass of the plunger, does not affect themagnetic properties of the plunger to a significant extent and that theplunger behaves magnetically as if it were a solid member. For example,the axial bore can be formed as a blind ended drilling in a solid rod ofa suitable material. Such a form of construction of the plunger is muchsimpler than the complex two component construction described in PCTApplication No SE97/01007. The amount of metal removed from the plungerdepends upon a number of factors, for example the strength required forthe resultant hollow plunger, the magnetic force required to move theplunger against the bias of the spring and the acceleration anddeceleration required at each end of the travel of the plunger. However,the bore should not extend significantly into that portion of theplunger which lies within the coil when the plunger is in its fullyretracted position. We have found that if the bore extends significantlybeyond this point, the magnetic coupling between the coil and theplunger, and hence the force driving the plunger at the initiation ofits extension stroke towards its extended position, is reduced.

Removal of material from the plunger reduces the mass and hence inertiaof the plunger. However, removal of material would have been expected toreduce the magnetic force which can be applied to the plunger by thecoil and hence to result in a plunger that could not be acceleratedsufficiently by the magnetic field applied to it. We have found thatthis is counteracted by the use of materials of construction for theplunger which have a high saturation flux density as described above.Preferably, the plunger/coil combination has a high magnetic inductance,typically 9 milliHenrys or more. Suitable materials for use as theplunger material include magnetisable iron or steel alloys, notablythose of iron and nickel containing from 40 to 55% of nickel, especiallythose containing from 45 to 50% nickel and from 55 to 50% of iron. Ifdesired, other metals such as chromium or aluminium may also be presentin minor amounts. Preferred materials for present use are those whichhave a saturation flux density in excess of 1.6 Tesla, for example 1.8Tesla or more. The preferred materials also have a coercivity less than0.25 amperes per metre, and a permeability in excess of 50,000, forexample 100,000 or more. Suitable materials for present use includethose ranges of alloys sold under the Trade Names Permenorm 5000 andVacofer SI. Where high Tesla materials are used, it may be desirable toprovide the plunger with a corrosion resistant external layer orsurface.

If desired, composite materials, such as polymers or sintered or frittedceramics or silicon matrices, which have particles of a suitableferromagnetic or other magnetisable material dispersed therein or whichhave otherwise been modified to have the desired high saturation fluxdensity and/or magnetic inductance may be used. For example, a plungermay be formed from laminates of materials of different properties toachieve the desired overall magnetic properties.

The plunger typically comprises a generally cylindrical member which ismade from a suitable magnetic material and which is a sliding fit withinthe coil or the tubular member within or upon which the coil issupported. The plunger can be of any suitable length and cross sectionwhich is preferably congruent to the internal cross section of thetubular support for the coil. However, it is particularly preferred toform the plunger with as a low a mass as is practicable so as to enhancethe speed of response of the plunger to changes in the electric currentapplied to the coil. Thus, the plunger may have an axial bore therein asdescribed above. Alternatively, the plunger may be formed as a polymer,ceramic or other matrix containing a suitable ferromagnetic materialincorporated therein. However, it is particularly preferred to form theplunger as a unitary solid body from a soft ferromagnetic material witha diameter of less than 2.5 mms, notably about 1 mm, and a length todiameter ratio of more than 3:1, preferably from about 5:1 to 10:1, sothat the plunger slides freely within the tubular support member of thecoil.

In a preferred embodiment, therefore, the invention provides a compact,light weight solenoid valve of the invention characterised in that theplunger has a diameter of less than 2.5 mms, notably about 1 mm, and hasa length to diameter ratio of more than 3:1, preferably from about 5:1to 10:1.

The plunger is conveniently formed by machining, rolling or extrudingthe desired alloy to form a length of material having the desired sizeand shape. During the machining of the preferred materials ofconstruction to form the plunger for the valve, the magnetic propertiesof the material may be affected. It may therefore be desired to subjectthe manufactured plunger to some form of post forming treatment so as torecover the magnetic values. Such treatments include heat treatment ormechanical impact treatment which cause a change in the crystalcomposition of the material. The optimum form of post forming treatmentcan be readily determined using simple trial and error.

The distal end of the plunger may be provided with a plastic or rubberend face which forms a fluid tight seal when it bears against thetransverse end wall of the valve head chamber. If desired, that end wallcan be provided with one or more upstanding circumferential ribs or sealmembers generally concentric with the inlet to the outlet bore of thevalve head chamber so as to enhance the sealing engagement of the endface of the plunger with the end wall of the valve head chamber.However, where the bore is provided by a length of capillary tube, theend of the tube can protrude into the valve head chamber to provide anannular sealing surface with which the distal end face of the plungerengages. The optimum form of the sealing arrangement can readily bedetermined by simple trial and error tests and a combination of two ormore forms of sealing arrangement may be used if desired. The raisedribs or other sealing arrangement can be formed using any suitabletechnique, for example during the moulding of the transverse end wall.Surprisingly, we have found that the use of annular raised sealing ribson the distal end of the plunger and/or on the transverse face of thevalve head chamber with which it engages has the effect of reducing theformation of satellite drops from the nozzle orifice, especially atoperation of the valve at frequencies in excess of 1 kHz.

The valve mechanism has a coil through which an electric current ispassed to generate the magnetic field which acts upon the plunger. Sucha coil can be of conventional design and construction and serves togenerate the magnetic field required to move the plunger when a currentflows through the coil. Preferably, the coil is a single winding of asuitable wire wound on a suitable bobbin which may be removed and theresultant free coil potted in a suitable resin to provide a radiallycompact coil having a central axial bore which provides the tubularmember within which the plunger reciprocates. If desired, the coil canbe wound with two or more taps so that different electric currents canbe applied at different points axially along the coil. For convenience,the invention will be described hereinafter in terms of a single coilhaving a single pair of connectors to connect to a source of current.

However, as indicated above, we prefer to minimise the radial gapbetween the coil and the plunger so as to optimise the magnetic couplingbetween the coil and the plunger. This can be achieved by winding thecoil directly upon the tubular member within which the plungerreciprocates, with a thin insulating interface between the wire of thecoil and the tubular member where a metal tube is used. Alternatively,the coil can be formed by winding a bare wire coil upon an insulatingtubular member and then retaining the coil in position by applying aretaining coat of resin or other binder upon the wound coil.Alternatively, the coil can be wound upon a mandrel, removed and thenpotted in a suitable resin which then forms the wall of the tubularmember within which the plunger reciprocates. In a particularlypreferred embodiment, the tubular member is formed from a ceramicmaterial, for example as a ceramic frit tube. The coil can be formed bydepositing a conductor track, for example by vapour phase or electricaldeposition of a copper or silver conductor or track, upon the surface ofthe fritted tube or into grooves etched, machined, laser cut orotherwise formed in the external surface of the ceramic tube.Alternatively, the coil may be formed as a copper, silver, gold or otherconductive track upon a flexible circuit board which is then rolled upona mandrel to form a cylindrical tubular member incorporating the coil.In all such designs the gap between the coil has been reduced, typicallyto a radial dimension of less than 1 mm, typically less than 0.5 mm, forexample 100 to 200 micrometres, as compared to the 2 mm or greater gapin a conventional solenoid coil. Such a reduction in the gap results ingreater efficiency in coupling the plunger magnetically to the coil,resulting in lower power consumption and greater speed of response ofthe plunger to changes in the current flowing in the coil. Suchconstructions also result in a unitary construction for the coil and thetubular member within which the plunger reciprocates, thus simplifyingconstruction and assembly of the valve and enables a more compactconstruction to be achieved. Furthermore, since a smaller driving forceis typically required to move the plunger in a valve of the invention,it is often possible to form the coil with a single layer of wire orother conductor. This further assists the formation of a compactconstruction.

If desired, the tubular member supporting the coil can be longitudinallyextended to provide the radial walls of the valve head chamber. In oneembodiment of such a construction, the tubular member is formed as acylindrical tube having one end closed to form the transverse terminalwall of the valve head chamber, the wall being pierced by a bore whosefree end provides the nozzle orifice. Such an assembly can readily beformed by electo or laser etching of a silicon or ceramic member to highaccuracy using automated techniques.

The valve mechanism of the invention is preferably used in co-operationwith a plurality of closely adjacent valve mechanisms, each serving oneor more discrete nozzle orifices, to form an array type print headcapable of applying a plurality of dots of fluid simultaneously to asubstrate to create a two dimensional image. Such an array can be formedby mounting the outlet end of the valves upon a nozzle plate with a borethrough the plate providing the outlet bore from the valve head chamberof the valve to the nozzle orifice. Typically, a jewel nozzle is setinto the nozzle plate to provide both the bore from the valve headchamber and the nozzle orifice. In a particularly preferred embodiment,the nozzle plate is provided with a series of upstanding tubes, each inregister with one of the bores through the plate. The tubes serve as thetubular member support for the coil of the valve and the plungerreciprocates within that tube. The distal end portions of the tubesadjacent the nozzle plate, or the proximal portion of the bore in thenozzle plate, serves as the valve head chamber of the valve mechanism.Such arrays can be formed from ceramic or silicon materials usingautomated techniques and the nozzle orifice can be provided either by ajewel nozzle set into a bore through the nozzle plate or by forming asuitable nozzle orifice in the end of a blind end bore in the nozzleplate using a laser. Such assemblies can be formed on a very small scaleenabling miniaturisation of the valve structure to be achieved whichaids the printing of small dots, typically less than 60 micrometres indiameter, on the substrate to be printed, enhancing the definition ofthe printed image.

It is also preferred to provide each valve mechanism with a metalhousing to the coil thereof. This acts not only as a return path for themagnetic field generated by the coil within it, but also acts as amagnetic screen so as to reduce cross talk between the magnetic fieldsgenerated by one coil and the coil of an adjacent valve mechanism.Typically, such a metal housing is made from .mu. metal, aluminium orstainless steel and also acts as a rigid housing for the components ofthe valve mechanism. Thus, the housing can be of a generally cylindricalform and can be crimped radially inwardly at each end thereof to retainend pieces and the coil axially clamped upon one another, one end piececarrying an axial fluid inlet, the other defining the valve head chamberand carrying an axial capillary tube or jewel nozzle which forms theoutlet bore between the chamber and the nozzle orifice. Alternatively,the distal end of the metal housing can be crimped or otherwise securedto the nozzle plate where the nozzle plate carries upstanding tubularmembers as described above.

The valve mechanism of the invention preferably also comprises a means,notably a spring, for biasing the plunger towards its rest position.Typically, the spring is a compression spring and acts to bias theplunger against the inlet at the proximal end of the bore to the nozzleorifice, so that the rest position of the plunger is in the valve closedposition. When a current is applied to the coil, this opposes the biasof the spring and moves the distal end of the plunger away from the boreinlet to open a flow path from the valve head chamber to the nozzleorifice. However, it will be appreciated that the rest position may bethe valve open position and the operative position is the valve closedposition. For convenience, the invention will be described hereinafterin terms of the rest position being the valve closed position.

The spring member is pre-tensioned, for example from 50 to 80% of thetravel of the compression of the spring is taken up by thepre-tensioning, since we have found that such pre-tensioning enables thespring to apply a consistent bias force against the movement of theplunger over the remainder of the compression of the spring duringmovement of the plunger. We have found that the use of a conical springis of especial benefit since such springs can readily be fitted withinthe dimensions of the valve head chamber and will tend to be selfcentring during the assembly of the valve mechanism, whereasconventional cylindrical coil springs do not. Furthermore, the use of aconical spring reduces the mass and hence inertia of the spring; furtheraiding rapid response of the spring to movement of the plunger. It isparticularly preferred to use a conical coil spring which ispre-tensioned to the last two turns of the spring, since we have foundthat such a spring responds rapidly to the movement of the plunger andthe pre-tensioning enables the spring to exert a significant bias forceover a small additional compression of the spring.

However, it will be appreciated that the bias effect could be appliedalternatively or in addition to that applied by the spring by applying acurrent to the coil which opposes the movement of the plunger. Suchopposing current can be applied under the control of electronicswitching using conventional techniques and software, for example asdescribed below.

The valve of the invention also comprises a valve head chamber in whichthe distal end of the plunger co-operates with the bore leading from thevalve chamber to the nozzle orifice to open and close a flow path forfluid to the nozzle orifice. This valve chamber may take the form of asimple cylindrical extension of the metal housing described above toprovide the magnetic screen between adjacent valves in an array ofvalves or of the tubular support member for the coil. A transverse endwall carrying the axially extending bore to the nozzle orifice issecured to that housing, for example by a crimping operation, to form aclosed chamber at the distal end of the coil. Alternatively, asdescribed above, the tubular support member can extend axially beyondthe coil to form the side wall of the valve head chamber. A transverseend wall can be formed integrally with or secured to the distal end ofthe tubular support member and the nozzle orifice formed in that endwall, for example by a laser.

The outlet to the valve head chamber can be provided by an axiallyextending tube, the proximal end of which passes through the transverseend wall and the distal end of which forms or is provided with thenozzle orifice. Where the outlet is provided by an axially extendingtube, for example a stainless steel capillary tube, this can be used tomount the valve assembly upon a nozzle plate or other support.Alternatively, the metal housing providing the magnetic screen aroundthe coil may be provided with lugs or other means for mounting the valvemechanism. Where the valve mechanism comprises tubular coil supportsextending transversely from a nozzle plate as described above, these andthe nozzle plate provide the means for mounting and securing the valvemechanism in the desired location.

In a preferred embodiment of the invention, the valve mechanism ismounted on a nozzle plate having a plurality of nozzle bores which areformed substantially simultaneously in a single operation so that thenozzle plate has a unitary construction without the use of jewelnozzles. Such a simple unitary nozzle structure can readily be madeusing a wide range of techniques and overcomes the problems associatedwith misalignment of jewel nozzles in a multi-nozzle print head. In sucha preferred embodiment, the nozzle orifice and bore through which theink is ejected upon operation of the valve are formed as a unitarystructure, for example concurrently as a bore is cut or otherwise formedin a plate upon which the valve mechanism is to be mounted. For example,the bore/nozzle orifice is formed in a nozzle plate by a laser,electro-forming or etching, needle punching or other techniques. Thenozzle plate can be from 50 to 400 micrometres thick so as to achievethe desired length to the bore. At such thickness, the nozzle platetakes the form of a metal or other foil which is mounted in a suitablesupport member to provide a rigid and mechanically strong nozzle plateassembly. We have found that by forming the nozzle bores simultaneouslyin a multi-nozzle nozzle plate, problems due to misalignment of thebores with one another are minimised.

We have also found that by selection of the bore forming technique, thewalls of the bore are sufficiently smooth to reduce flow separation andthe formation of eddies at the interface between the bore walls and thefluid flowing through the bore. Furthermore, such techniques may also beused to form other features on the nozzle plate which enhance theoperation of the valve. For example, electro-forming or etching of ametal foil can be used to form the bores/nozzle orifices in the plateand also to form a raised lip or ridge around the inlet to the boreleading to the nozzle orifice. This provides a localised pressure pointbetween the distal end face of the plunger and the nozzle plate toassist the formation of a fluid tight seal when the plunger is in thevalve closed position. Alternatively, where a needle is used to form thebore in a metal foil, this will cause the foil to deform and form abelled entry to the bore which will assist smooth flow of fluid into thebore from the valve head chamber. The penetration of the needle throughthe foil may also polish the surface of the foil, and hence the internalwall of the bore which is formed, as the surface of the needle slidesover the material of the foil. Similarly, the use of a laser to form thebore in a metal, ceramic or plastic foil may also form a polishedsurface to the walls of the bore, notably where the laser beam is pulsedfor very short periods, typically less than 1 nanosecond, to reduce theformation of deposits around the lip of the bore of material which hasbeen ablated from the plate in forming the nozzle bore.

Such assemblies can be formed on a very small scale enablingminiaturisation of the valve structure to be achieved. It is preferredto provide the nozzle plate as a metal, ceramic or other foil having thebores formed therethrough as described above and to mount that plate sothat the bores therein are in register with the distal ends of theplungers of the valves. In this case, the valve head chambers can beindividually formed in the surface of the foil or in an intermediateplate located between the valve coil support members and the nozzleplate. However, we have found that the flow of ink or other fluid to theindividual bores and nozzle orifices is enhanced if the intermediateplate is formed with a continuous chamber which provides a combinedvalve head chamber for all the valves in the print head assembly. Insuch a construction, the seal between the distal end face of eachplunger and the registering bore in the nozzle plate provides adequateisolation of flow through each of the nozzle bores and orifices.

If desired the raised sealing ribs or areas on the nozzle plate can beformed from a flexible material to cushion the impact of the end face ofthe plunger against the nozzle plate. Such deformation may also assistformation of the fluid tight seal where the end face of the plunger doesnot carry a rubber or similar pad. If desired, the pad carried by theend face of the plunger can be formed from a material which undergoescold creep or deformation under the load of the bias spring urging theplunger into the valve closed position. Such creep may form a nipple orother projection which extends into the proximal portion of the nozzlebore in the nozzle plate. Upon reciprocation of the plunger, thisprojection repeatedly wipes at least the initial part of the proximalportion of the nozzle bore and displaces solid deposits which may havedeposited upon the wall of the bore and this may assist in reducinginitial drop deformation after rest periods of the valve. To assist theoperation of this projection, the mouth to the inlet to the bore throughthe nozzle plate may be belled, as may occur when a needle is used toform the bore in the nozzle plate.

Fluid can be fed to the valve head chamber by any suitable means, forexample by one or more radial inlet ports in the side wall of thechamber. Alternatively, fluid can be caused to flow axially past part orall of the plunger within the coil so that the fluid lubricates themovement of the plunger within the coil and can also act to cool thecoil at high current loadings and/or high frequencies of operation ofthe valve. Thus, the bore in the tubular support for the coil can have agenerally circular cross section and the plunger may have a squared orhexagonal cross section, axial flattenings, grooves or flutes which formaxial fluid flow paths along the plunger. Where this is done, theproximal end of the valve mechanism can be provided with an axial inletto feed fluid axially into the space(s) between the plunger and thecoil.

Such a valve is capable of being operated at high frequencies, typically2 to 9 kHz and finds especial application as the solenoid valve in adrop on demand ink jet printer head. In such an application, the valveis desirably as small and compact as possible so as to reduce theoverall size of the print head and the inertia of the components of thevalve mechanism. The valve is incorporated into any suitable form ofdrop on demand printer to control the flow of ink through the nozzleorifices of that printer. As described above, the valve mechanism can beincorporated into a compact structure forming an array print head whichis operated under the control of a computer which determines the opentime of each of the valves and the sequence of opening of the valves toprint the desired image. Such a computer can operate in the conventionalmanner. However, as described below, the computer may be used to achievemany other functions in the control of the operation of the printer.

Accordingly, from another aspect, the invention provides a drop ondemand printer characterised in that the flow of ink through the nozzlesof the printer is regulated by a valve of the invention.

For convenience, the invention has been described in terms of such useof the valve mechanism. However, it will be appreciated that the valvemechanism of the invention may be used wherever a small, high speedvalve is required, for example in the dosing of measured amounts of areagent in a chemical or biological analysis or other process, notablyin the assessment of medicaments or in diagnostic testing or analysis.The valve of the invention also finds use in applying a predeterminedquantity of a reagent in the verification of the authenticity of asample.

Surprisingly, we have found that the application of droplets of ink athigh frequencies to long pile fabrics, for example carpets and felted orwoven fabrics, enables satisfactory application of the dye to the fibrewithout the need to use very high viscosity inks. Thus, in place of inkshaving viscosities in excess of 250 Cps hitherto considered necessary toachieve good dying of the fibres, we have found that good results can beachieved using inks of from 60 to 120 Cps applied at frequencies ofabout 2 kHz. The ability to use low viscosity inks enables the printingto be achieved using smaller nozzle orifices, which increases thedefinition of the pattern printed on the fabric. It also enables theoperator to select inks from a wider range than hitherto and to operatethe printer at lower ink pressures, which reduces the need for specialmodification of the printer and the risk of failure of components.

Many fabrics, both woven and non-woven, have a surface which presentsfree ends of fibres generally normal to the plane of the fabric. Suchfabrics include felted materials where fibres in a randomly orientatedmass are compressed, optionally in the presence of a bonding agent suchas an adhesive; materials woven from strands made up from a plurality ofindividual fibres where the surface of the fabric has been, brushed,teased, abraded or otherwise treated to separate some of the fibres fromwithin the strands to form a fluffy surface to the material, for examplea brushed nylon; woven materials made from materials which areinherently fluffy, such as knitted or woven angora, merino or cashmerewools or cotton terry towelling; and carpet type materials such asvelvets, velours and tufted carpets where individual lengths of strandsor fibres are knotted, sewn, glued or otherwise secured to a sheetmember, typically a reticulate backing sheet, whereby the free ends ofthe strands or fibres form a pile which extends generally normal to theplane of the backing or where loops of the strands or fibres are formedextending generally normal to the plane of the backing and the free endsof the loops severed to form the pile. For convenience the term pilefabric will be used herein to denote all such types of material whereindividual fibres or strands comprising groups of fibres extendgenerally normal to the plane of the material to provide a pile effectsurface to the material.

It is often desired to form patterns or images upon the surfaces of pilefabrics, for example a coloured pattern. This can be achieved byinterweaving different coloured, textured or other material strands ofwool or other material into the fabric as it is being made. However,this is difficult and time consuming, especially where the pattern iscomplex and/or a plurality of colours or textures are desired. Such useof a plurality of different strands is becoming progressively uneconomicin the large scale manufacture of commodity materials, such as patternedcarpets.

It has therefore been proposed to manufacture the pile fabric fromneutral or uniformly coloured fibres or strands and to apply a colour tothe pile fibres after the fabric has been woven or otherwisemanufactured. The colour is typically an ink applied by any suitableprinting technique. A printing technique which is used is an ink jetprinting technique using a drop on demand type of printer. The ink isdesirably applied at the rate of about 300 to 400% by weight of thefibre to be coloured and needs to penetrate substantially uniformlythroughout the strands formed from the individual fibres. If a verymobile ink having a viscosity of about 10 cPs at 25° C. (as is commonlyused in an ink jet printer) is used, it will run down the length of thestrands and form an intense coloration at the base of the pile, leavingthe top portion of the pile inadequately dyed, and little penetration ofthe colour into the strands will take place. It is therefore necessaryto increase the viscosity of the ink in order to ensure that it runsdown the fibre at a sufficiently slow rate for uniform penetration ofthe ink into the strands and coverage of the surface of the individualfibres takes place. The longer the pile, the greater this problembecomes. With long pile fabrics, that is those with a pile length ofabout 2 mms or more, it is necessary to use inks having a viscosity offrom 250 to 500 cPs at 25° C.

Such viscous inks are difficult to jet through the very fine orificenozzles of a conventional ink jet printer and pressures far in excess ofthose for which the printer is designed would be required. Furthermore,if a low viscosity ink were applied at such high pressures, it wouldissue from the nozzles as high powered jets and cause the individualstrands to bend over and thus prevent the ink from contacting otherstrands in the pile. It is therefore customary to use nozzles havingorifices which are progressively greater as the length and closeness ofthe pile increases. Thus, for a carpet having a pile length of 3 mms ormore it is necessary to use an ink having a viscosity of about 300 cPs,a pressure of about 2 bar and nozzle diameters of typically 500micrometres in diameter so that the viscous ink can be ejected insufficient amounts to attain the desired loading of ink on theindividual strands.

Whilst the use of large diameter nozzles for high viscosity inks enablesthe ink to be deposited on the strands of the pile to achievesubstantially uniform coloration of the individual strands and fibres,the size of the droplets issuing from the nozzle are sufficiently largeto cause perceptible loss of definition in the printed pattern.Furthermore, the size of the droplets also results in adjacent dropletsapplied to the pile contacting one another to cause colour bleedingwhere the droplets are of different colours.

Surprisingly we have found that the use of a drop on demand print headwhich operates at frequencies greater than 1 kHz, notably a print headincorporating a valve of the invention, enables the size of the dropletsbeing printed and hence the pressure required to eject them throughcomparatively small nozzle orifices to be reduced. This reduces theproblems of colour bleed and enhances the definition of the printedimage or pattern. Furthermore, we have found that it becomes possible toomit individual printed droplets from the printed pattern and thus printa blank area within the image which is not visually perceptible butwhich acts to provide a gap within the printed strands to act as abarrier to colour bleeding. Such a gap may also be printed as a blackline defining the edges of areas printed with different colours, whichenhances the perceived definition of the printed image or pattern.

Accordingly, from another aspect, the present invention provides amethod for applying an image forming composition to a pile fabric usinga drop on demand ink printer, characterised in that the printer isoperated at a drop generation frequency of at least 1 kHz. Preferably,the pile fabric has a pile length of at least 2 mms and the printer isoperated at a pressure of less than 3 bar, notably at from 1.5 to 2.5bar.

A particularly preferred drop on demand ink jet printer is one utilisinga valve of the invention to control the flow of an ink through theindividual nozzle orifices and the nozzle orifices have a diameter offrom 250 to 600 micrometres, notably about 500 micrometres; and in whichthe plunger of the solenoid valve has a diameter of less than 2.5 mms.We have also found that the use of such a printer enables individualcontrol of the printing of the dots of the image so that accurateover-printing of dots can be achieved. It is thus possible to enhancethe colour range and strength which can be achieved. Such a printer thusenables an infinite scaling of the colour hues which can be achieved.

The invention can be applied to the application of any form of image toany pile fabric. However, the invention is of especial application inthe application of a water and/or solvent based ink composition to forma patterned image on a long pile fabric having a pile length of about 2to 5 mms as measured from the top surface of the sheet member to whichthe strands or fibres forming the pile surface of the fabric aresecured. Such pile fabrics can be velvets or twist pile carpets, but forconvenience the this aspect of the invention will be described in termsof printing a multi-colour pattern on a tufted pile carpet in which thestrands containing a plurality of individual fibres are secured to areticulate backing sheet by adhesive. Such carpets can be made by anysuitable technique and the invention can be applied during thefabrication of the carpet after the strands have been secured to thebacking sheet or can be applied after the carpet has been manufacturedin a separate colour printing operation. As indicated above, the strandsare made from a neutral tint fibre, for example from a natural washedwool fibre, optionally in admixture with one or more natural colouredpolymer fibres, for example polyester or polyamide fibres. If desired,the fibres or the strands formed from the fibres may be given one ormore treatments to render the fibres receptive to the ink composition tobe applied to them. The fibres, their formation into strands, thetreatment of the fibres or strands and the formation of the carpet canall be those conventionally used in the manufacture of a tufted carpet.

For convenience, the this aspect of the invention will be describedhereinafter in terms of the application of ink to a neutral washed woolfibre tufted carpet shortly after the pile has been formed on areticulate woven polypropylene backing sheet.

In a preferred embodiment, this aspect of the invention, the printer isone in which the solenoid valve mechanism for controlling the flow offluid to the nozzle orifice comprises a plunger member journalled foraxial reciprocation between a rest and an operative position within anelectric coil under the influence of a magnetic field generated by thatcoil when an electric current passes through the coil, the distal end ofthe plunger extending into a valve head chamber having an outlet nozzlebore, the reciprocation of the plunger being adapted to open or close afluid flow path from the valve head chamber through that bore,characterised in that:

a. the plunger is of a unitary construction and is made from anelectromagnetically soft material having a saturation flux densitygreater than 1.4 Tesla, preferably about 1.6 to 1.8 Tesla, a coercivityof less than 0.25 ampere per metre, and a relative magnetic permeabilityin excess of 10,000; and

b. the nozzle bore leading from the valve head chamber to the nozzleorifice has a length to diameter ratio of less than 8:1, preferably from1.5:1 to 5:1, notably from 2:1 to 4:1.

The term magnetically soft is used herein to denote that the materialloses the magnetic field induced in it by the coil when the current inthe coil ceases, in contrast to a permanent magnet which retains itsmagnetism.

The use of materials having high magnetic flux saturation densitiesenables the plunger to respond rapidly to changes in the magnetic fieldgenerated by the coil without the generation of excessive heat. The lowcoercivity of the plunger material also aids the rapid rise and fall ofthe induced magnetic field within the plunger under the influence of thefield generated as a current is passed through the coil at low appliedcoil currents. This, coupled with the high permeability of the material,enables a high magnetic drive force to be generated rapidly between thecoil and the plunger. As a result, the plunger can be acceleratedrapidly by the coil without the need to apply high drive currents to thecoil, typically in excess of 20 amperes, as hitherto considerednecessary. This again reduces the heat energy which is generated as theplunger is moved by the coil. The low coercivity also permits a reversemagnetic force to be generated rapidly by reversing the direction of thecurrent in the coil. This reversed force can be used to slow down themovement of the plunger as it reaches either or both extremes of itstravel as described below.

Furthermore, we have found that the above design of valve can be held inthe open position for prolonged periods to print continuous lines on thesubstrate which have a length equivalent to at least three individualprinted dots. With conventional solenoid valves, it has been considerednecessary to pulse the current to the coil so as to form overlappingdots of ink on the substrate. In practice this often leads to the valvesburning out due to the high currents applied to the coil to move theplunger from its initial rest position into the valve fully openposition. We have found that the amplitude of the current flowingthrough the coil required to hold the plunger in the valve open positionis surprisingly much less, typically 80 to 50% less, than the currentrequired to move the plunger initially away from its rest position. Byapplying a current pulse which has an initial amplitude sufficient tomove the plunger from its rest position to the valve open position andthen reducing this amplitude to a lower value for the remainder of thepulse, it is possible to hold the valve open for prolonged periods so asto print lines of ink on the substrate.

Furthermore, by reducing the length of the nozzle bore, the pressuredrop across the nozzle is reduced, allowing a faster exit velocity to beachieved at the nozzle orifice. Surprisingly this is achieved withoutcausing spraying of the droplets, that is the break up of the droplet atthe nozzle orifice into a plurality of smaller droplets. This enables ahigher frequency of droplet generation to be achieved at a given inkpressure for a given length of flight path.

In a conventional drop on demand printer, the operation of each solenoidvalve is triggered in response to a signal from a computer ormicroprocessor, which determines which valve is opened and when so as toprint the desired image. We have found that the control the operation ofthe valve using software has a number of other significant benefitswhich enable the valve of the invention to deliver high quality printedimages at much higher frequencies that has hitherto been consideredpossible for a drop on demand print head.

Thus, it is particularly preferred to use software to calibrate thevalve so that under specific conditions it delivers a consistent dropletof ink through the nozzle orifice. With conventional designs of solenoidvalve, it is necessary to compensate for minor variations in dimensionsand materials of the manufactured valve by physically adjusting theaxial travel of the plunger so as to vary the size of the flow pathcreated when the plunger is withdrawn from sealing engagement with thetransverse end wall of the valve head chamber or the tube leading to thenozzle orifice. This will affect the size of the dot ejected from thenozzle orifice and the objective of the calibration process is toachieve a uniform droplet size from all the nozzle orifices in a printhead under the same printing conditions. The conventional design ofsolenoid valve incorporates a stop within the bore of the tubularsupport for the coil, which stop provides a physical limit to the axialmovement during retraction of the plunger. In such a conventional valvedesign, the air gap between the proximal end of the plunger and the stopis adjusted, for example by making the stop a stiff push fit or a screwfit within the tubular support, so that it can be moved axially withinthe bore of the tubular member to achieve the desired air gap. Suchadjustment of the air gap is tedious and time consuming and is prone tooperator error.

We have found that software can be used to set a specific point in theretraction of the plunger at which the plunger movement halts. Thispoint can readily be adjusted by simple modification of a parameter ofthe software, for example by keyboard input of a new value for thatparameter. Such adjustment can be achieved very accurately and thecalibration carried out for a number of sets of printing conditions sothat the current pulse size and duration required to achieve givendroplet sizes can be determined and stored, for example in as a machinereadable code on a magnetic disc, look up table in a memory chip orother storage medium, for future use with that valve. The calibrationcan be achieved simply and at smaller increments of droplet size than ispossible with screw adjustment of the stop in conventional design ofsolenoid valve.

In carrying out the calibration, droplets are printed onto a substratewhilst operating the valve under standard conditions and at a givenelectric current pulse amplitude and duration applied to the coil. Theprinted dot is examined by any suitable means and the amplitude and/orduration of the electric pulse raised or lowered to achieve the desireddot size. Such a process can be carried out manually. However, it ispreferred to carry out this process automatically by inspecting theprinted dot using a CCD camera or other inspection means and comparingthe form of the printed dot with parameters for the required dot. Suchcomparison and subsequent adjustment of the current pulse can be carriedout using a suitably programmed computer. It is especially preferred tomonitor the diameter and circularity of the printed dot and the presenceof satellite small dots adjacent the desired dot using a CCD array orcamera and comparing the dot characteristics with those held in a lookup table which identifies the correction which needs to be applied tothe current pulse applied to the coil to achieve the desired printed dotcharacteristics. The optimum variation in the operation of the valvemechanism, for example to increase or reduce the open time of the valve,can be determined by trial and error tests. These optimum values of thevariation then stored in a look up table or other storage medium toprovide one of the parameters against which the printed dot and theoperation of the print head is assessed.

The use of a CCD camera or array and computer to inspect the droplet ofink as it is ejected and/or the printed dot and to modify the currentapplied to the coil of the valve also has applications during theoperation of the valve on-line during printing of images. Thus, thecomputer can be programmed to decelerate the movement of the plunger ateach end of its travel. We have found that this reduces splatter of theink from the nozzle orifice due to sharp impact of the plunger againstthe seal members at the entry of the bore between the valve head chamberto the nozzle orifice. The use of software can also be used tocompensate for fluctuations in the viscosity of the ink due totemperature variations or other reasons; variations in voltage appliedto the different coils in an array of print heads which are operatedsimultaneously; and to compensate for other changes in operatingconditions, for example the use of a different ink, which requirechanges in the form and size of the electrical pulse applied to the coilof the valve. The use of software can also be used to hold a valve inthe open position to print a continuous line of ink in place of theseries of overlapping dots achieved with present print head operatingtechniques; and to vary the open time of the valve for the initialdroplets ejected from the nozzle orifice following a rest period of thevalve.

In all cases the operation of the valve is modified by the computer inresponse to a signal from the CCD camera or other mechanism used toinspect and monitor the droplet of ink as it is ejected and/or theprinted dot and to compare the observed droplet or dot to parametersheld in a memory of the computer or another storage medium so as todetermine what modification, if any, is required to the current appliedto the coil so as to achieve the desired dot.

The invention thus provides a print head of the invention operated underthe control of a computer in combination with a mechanism for observingthe printed dot of ink or other fluid applied to the substrate, thecomputer being programmed to detect differences between the observed dotand the desired dot and to apply a correction to the current applied tothe coil so as to maintain the desired observed dot parameters.

Such a combination enables the printed dot quality to be monitored andcorrected on-line during operation of the printer. Hitherto, the printquality was observed objectively by the operator of the printer and acorrection to the operation of the printer applied manually. The abilityto use the software on-line to achieve monitoring and correction ofprint quality is a major benefit to the operator and can achievevirtually instantaneous correction of fluctuations in print quality.

The monitoring and correction may be achieved using conventionalsoftware and hardware techniques and designs. The dot quality can bemonitored continuously and a correction applied in response to theaverage of three or more successive dots. Alternatively, the printed dotquality can be monitored at intervals, for example every second or atintervals of every twenty operations of the valve, and any correctionapplied once the printed dot deviates by more than say 5% for any one ormore of the parameters used to assess the quality of the printed dot.

Typically, the monitoring of the printed dot quality will be used toapply a signal to vary the open time of the valve.

It will be appreciated that the signal indicating that some variation ofthe operation of the valve is required may be provided from an externalsource rather than from the on-line scanning of the printed dot. Thus, asensor may monitor the operating temperature of the printer and/or ofthe ink fed to the valve, since this will affect the viscosity and hencethe jettability of the ink. Alternatively, such sensors may monitor: thevoltage applied to the valve mechanism, for example the voltage dropwhich occurs when a plurality of valves are operated simultaneously froma single power source; the time for which a specific valve has restedbetween printing operations, the frequency of operation of a valve andso on. These sensors may then address a series of look up tables whichthen set the variation of the open time required to reduce defects inquality of the printed dot if that parameter being monitored varies froma predetermined optimum value.

It is preferred that the quality of the printed dot from each nozzle bemonitored individually. However, if desired the printed dot quality fromgroups of nozzles may be monitored collectively.

In the conventional computed control of the operation of a valve in adrop on demand printer, simple single bit signals are used to open andshut the valve since all that has been required hitherto is that thecomputer instruct the valve when to open and shut the valve so as toprint a dot of the required size. However, the ability to vary theoperation of each valve individually during the operation of the printerin response to many inter-related factors requires the transmission ofmore complex signals than simple open and shut instructions. We havefound that it is desirable to transmit signals in byte format so thatthe amount of information transmitted can accommodate the permutationsof operating parameters desired. Thus, for example, the use of byte formsignal transmission offers 256 possible graduations of open time of thevalve. This enables the amount of ink deposited in each printed dot tobe varied over a finely graduated range by providing a look up tablewith 256 individual addresses therein from which the computercontrolling the operation of the printer can instruct the open time ofthe valve to be selected. This enables a true grey scale image to beprinted using a drop on demand print head, which has not hitherto beenconsidered practical. The use of byte signal transmission enables a wideselection of values for variation of a given operating parameter to betransmitted and responded to rapidly and accurately, further enhancingthe speed and accuracy of operation of the print head. This, coupledwith the high frequency printing of consistent quality dots over a widerange of sizes and speeds enables the present invention to extend theuse of drop on demand printers into fields of use for which they havehitherto not been considered possible whilst retaining the flexibilityof printed dot size which cannot readily be achieved with other forms ofprinter.

DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the invention and its operation under on-linesoftware control will now be described by way of illustration only andwith respect to the accompanying drawings, in which

FIG. 1 is a diagrammatic axial cross section through a valve of theinvention;

FIG. 2 is an axial cross section through a drop on demand ink jet printhead incorporating an array of the valves of FIG. 1;

FIG. 3 is plan view of the nozzle plate of the print head of FIG. 2;

FIG. 4 is a diagrammatic block diagram of an array of FIG. 2 incombination with a CCD camera for monitoring the quality of the printeddots and a computer for establishing what variation to the frequency,form, shape and amplitude of the electrical pulse applied to the coil ofthe valve of FIG. 1 is required to compensate for any deviation in thequality of the observed printed dot;

FIGS. 5 to 7 illustrate variations in the construction of the valve ofFIG. 1;

FIG. 8 illustrates an alternative form of the print head of FIG. 2;

FIG. 9 shows a schematic depiction of a solenoid valve which is suitablefor use with the calibration of the valve using software according tothat aspect of the present invention;

FIG. 10 shows in diagrammatic form an apparatus for use in this aspectof the invention;

FIGS. 11 to 13 illustrate alternative forms of the apparatus of FIG. 10;

FIGS. 14 and 15 show diagrammatically a valve and printer in which thecurrent applied to the coil is modified to decelerate the plunger ateither extreme of its travel; and

FIG. 16 illustrates the form of current pulse applied.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The valve of FIG. 1 comprises a plunger 1 which is journalled as a closefree sliding fit for axial reciprocation in a stainless steel tube 2.Tube 2 has a thin insulating coating or sleeve (not shown) formed uponits outer face and supports a coil 3 wound upon it. Coil 3 is suppliedwith an electric current from a source (not shown) under the control ofa computer 20, shown in FIG. 4. A stop 4 is mounted at the proximal endof tube 2 to limit the axial retraction of plunger 1 within tube 2. Thecoil 3 is encased in a metal cylindrical housing 5.

The above assembly is mounted in a support housing 10 which extendsaxially beyond the distal end of the coil and has a transverse end wall11 which carries a jewel nozzle 12. In the embodiment shown in FIG. 1,housing 10 has an axially extending internal annular wall 13 which formsthe radial wall of the valve head chamber 14 into which the distal endof the plunger extends. The distal end of the plunger 1 carries aterminal rubber or other sealing pad 15 which seats against the proximalend face of jewel 12 in sealing engagement. A pre-tensioned conicalspring 16 biases plunger 1 into sealing engagement with the face of thejewel as shown in FIG. 1, the rest or valve closed position.

Plunger 1 is made from a ferromagnetic alloy having a saturation fluxdensity of 1.6 Tesla such a Permenorm 5000 or similar magnetically softferromagnetic alloy. In order to reduce the mass of the plunger 1, itmay have a blind internal bore extending from the distal end thereof.However, this bore should not extend beyond line A-A shown in FIG. 1when the plunger is in its rest position. It is also desirable that theplunger have a diameter of less than 3 mms, typically about 1 mm, and alength to diameter ratio of about 5:1. For example, the bore in thejewel nozzle shown in FIG. 1 has an l:d ratio of 2 to 3:1 and the nozzleorifice has a diameter of 60 micrometres.

Ink is fed under a pressure of 1 bar to an ink gallery 17 encompassingwall 13 and enters the valve head chamber via radial ports 18. When theplunger is in its rest position as shown in FIG. 1, the pad 15 is insealing engagement with the face of the jewel nozzle 12 and thusprevents flow of ink through the nozzle orifice. In order to enhance theseal between the pad 15 and the jewel 12, we prefer to provide theproximal face of the jewel with one or more raised annular sealing ribs(not shown). This has the surprising effect of reducing satellitedroplet formation when the valve is operated at high frequencies,typically in excess of 1 to 2 kHz.

Such a valve can be operated at frequencies of from under 1 kHz to over8 kHz to produce consistently sized droplets in the size range 20 to 150micrometres or more by controlling the length for which the currentflows in the coil 3 and the frequency at which such current pulses areapplied to the coil.

As indicated above, the valve is preferably used in an array with othervalves to form a print head which extends transversely to the line oftravel of a substrate upon which an image is to be printed. Such anarray is shown in FIGS. 2 and 3. In this case the terminal portion 11 ofthe housing 10 is provided by a trough-shaped nozzle plate 30 carryingthe nozzles 12 and serving as a manifold to form the ink flow gallery 17feeding ink from ink inlet spigots 31 at each end of the nozzle platevia the inlet ports 18 to the individual valve head chambers 14 of thevalves in the array. The array is provided with a connector 32 by whichindividual electric current supplies can be fed to the coils 3 in eachof the valves. In such an array, the housing 4 serves to reduceelectrical and magnetic cross talk between adjacent valves in the array.

Such valves and arrays can be made by machining appropriate metalcomponents. However, one alternative form of construction is to form thetube 2 as a ceramic or silicon member 40 as shown in FIG. 5. The coil 41can be formed in grooves 42 cut in the external surface of the tube 40so that the air gap between the coil and the plunger 43 journalledwithin the tube is reduced. The coil 41 can be a wire coil wound intothe grooves 42; or can be a conductive track which is deposited by anysuitable means in the grooves 42. If desired, the assembly can then becoated with a polymer to retain and protect the coil within the grooves.In place of a rigid ceramic or silicon support tube, the tube 40 can beprovided by a sheet of a flexible support medium, for example a suitablefibre filled polymer or the like, upon which a copper or otherconductive track has been formed. The support medium is then rolled intoa cylinder to form a cylindrical support carrying the coil upon itsinner or outer face. In such designs, the tube 40 can extend axially toform the radial walls 44 of the valve head chamber and the distal openend of the tube closed with a jewel nozzle 45. The whole assembly canthen by encased in a stainless steel or other tube 46 which acts tosupport the assembly and provide the magnetic return path as screeningfor the coil. The ends of the tube 46 can be inwardly crimped to securethe tube 40, the coil 42 and the jewel 45 in position.

In place of the above forms of construction, an assembly of valves canbe formed as shown in FIG. 6 by forming a nozzle plate 50 from a siliconor ceramic frit or other material. This plate is provided with jewelnozzles 51 at the desired spacings along the plate 50. Plate 50 isprovided with upstanding tubular members 52 which form the tubes 40 ofthe valve design of FIG. 5. The coils 53 are wound or otherwise formedupon the upstanding tubular members 52 and the array is completed as inFIG. 5. The valve head chamber 54 is formed by the terminal distalportions of the tubular members and radial ink inlet ports may beprovided to enable ink to flow into the valve head chamber. A plunger 55is journalled in tubular members 52 for axial reciprocation under theinfluence of coil 53. In place of the jewel nozzle forming the closeddistal end to the valve head chamber, the plate 50 can be provided as acontinuously extending plate so as to form closed ends to the upstandingtubular members 52. These closed ends can then be pierced, for exampleby a laser, to form the bores therethrough and the nozzle orifices.

In place of the radial ink inlet ports to the valve head chamber 14 or54, ink can flow axially past the plunger 1 or 55 from an ink inlet tothe axially extending space between the tubular members 2 or 52 and theplungers 1 or 51. To form the axial passages past the plunger, the borein tubular member 2 or 52 can have an oval or polygonal cross sectionand plunger 1 or 55 has a circular cross section. However, it ispreferred to form plunger 1 or 55 with axial flats to it which provideaxial passages between the plunger and the circular cross section boreof the tubular member as shown in FIG. 7.

As indicated above, the operation of the valve is controlled by acomputer 20 in response to a CCD camera or array 21 or other sensors 22detecting the quality of the printed dots and/or other factors such astemperature, voltage, frequency of operation of the valve which alsoaffect the printed dot quality. Thus, the computer 20 determines whichvalve to open in the array of FIG. 2 and for how long so as to print adrop of the desired size at the desired position on the substrate 23passing the print head 24. At slow frequencies of operation, for examplebelow 1 kHz, this will usually result in a good quality dot beingprinted on the substrate. However, as the frequency increases, say to 2kHz or more, the quality of the printed dot may suffer, for example dueto the sudden closure of the valve causing the formation of satellitedots. The computer can respond to this by detecting from the CCD arraythat such satellite dots are being formed and causing the shape of thepulse of electric current applied to the coil to change so that themovement of the plunger at each extreme of its travel is reduced so asto reduce the sudden-ness of the closure of the valve by causing theplunger to soft land against the face of the jewel nozzle or end wall ofthe valve head chamber. Alternatively, the computer can respond to theinstruction to print at high frequencies by reducing the open time ofthe valve by reference to a look up table 25 which carries a list ofreductions in open time for a range of operating frequencies. Similarly,the software controlling the operation of the print head can detect whena valve has been idle for any length of time and provide, throughanother look up table, a signal to increase the open time of the valvefor the initial dots printed by that valve to compensate for any dryingout of the ink within the valve and/or at the nozzle orifice. In suchcases, it is preferred that the information between the computer and thelook up table be exchanged as bytes sized signals so that up to 256possible permutations of open time and operating frequency can beaccommodated in a single signal.

The printer of FIG. 2 was operated with a nozzle bore having a length todiameter ratio of 10:1, 8:1, 4:1 and 0.5:1 and at a drive currentfrequency of 2 kHz. At the 10:1 ratio, the pressure required to feed theink through the bore to achieve a consistent printed dot size was about10 Bar. However, such a pressure is too high for use with conventionaldrop on demand print heads and would have resulted in rupture ofcomponents. If the pressure was reduced to a more acceptable level, sayabout 3 Bar, the rate of flow of ink through the print head wasinsufficient to provide ink to form the droplets consistently so thatthe printed dots were of uneven size and there were missing dots wherethe valve had not been able to acquire ink from the reservoir.

Where the ratio was 8:1, the pressure required to feed the ink to thenozzle bore to achieve uniform printed dot size and quality was 5 Bar,which is at the upper extreme of operating capability of the componentsof a drop on demand printer.

Where the ratio was 4:1, the printer operated successfully at an inkpressure of 1.5 to 2.5 Bar and could print consistent dots at coil drivecurrent frequencies of from less than 1 kHz to 7 kHz.

Where the ratio was 0.5:1, the printer could not be operated, even atink pressures of 0.1 Bar without causing spraying of the ink and theformation of multiple small dots as well as the desired main dots.

The use of a preferred form of drop on demand print head as shown inFIG. 8 in printing images on a carpet pile fabric having a pile lengthof 3 mms under on-line software control will now be described by way ofillustration only.

The printer shown in FIG. 8 is a modification of the print head shown inFIG. 2 in which The nozzle plate 100 is formed with a plurality of bores101 therethrough having a length of 1000 micrometres and a diameter of500 micrometres. The plate is made from stainless steel and the boresare formed either by needle punches or by laser drilling each hole.Alternatively, the bores 101 can be formed by electro-etching, whichtechnique can also be used to form the raised annular ridge 102 aroundthe inlet to each of the bores 101. This foil nozzle plate is clampedbetween two stainless steel support plates 103 and 104. Plate 104 isformed with a single manifold chamber 105 which extends over all thebores 101 formed in plate 100. Alternatively, the chamber 105 can beformed in plate 100.

A valve assembly 110 contained in a support housing 111 is secured toplates 100, 103 and 104 with each of the plungers in a valve mechanismwithin the assembly in register with a bore 101. The valve mechanismscomprise a coil wound upon a support tube 112 within which a plunger 113is a loose sliding fit. Each coil is surrounded by a stainless steelhousing 114 which is crimped to an apertured support plate 115 clampedbetween housing 111 and plate 104 to locate and secure each valvemechanism with the plunger projecting through the aperture in registerwith a bore 101 in the nozzle plate 100. The electrical contacts for thecoils are fed from a multi-contact plug and socket from a computercontrolled power source, not shown. The valve head chamber for eachvalve mechanism 110 is provided by the single manifold chamber 105 whichis fed with ink from each end of plate 104.

The plungers 113 are made from a ferromagnetic alloy having a saturationflux density of 1.6 Tesla, a coercivity of 0.2 a/m and a relativemagnetic permeability of 100,000. The alloy is a 45/55 Ni/Fe alloy soldunder the trade mark Permenorm 5000 and each plunger is 2 mm in diameterand 7.5 mms long. The nozzle bore and orifice in the jewel nozzle have adiameter of 300 micrometres and an l:d ratio of from 2:1 to 3:1.

Ink having a viscosity of 250 cPs is fed under a pressure of 1.5 bar tothe manifold chamber 105 and enters the bore 101 when the plunger 113 isretracted by applying current to coil 112.

Such a valve can be operated at frequencies of from under 1 kHz to over8 kHz to produce consistently sized droplets in the size range 250 to500 micrometres by controlling the length for which the current flows inthe coil 112 and the frequency at which such current pulses are appliedto the coil.

The print head of FIG. 8 was used to apply inks having a viscosity of300 cPs through a nozzle orifice of 500 micrometres to apply differentcoloured inks to the pile of a neutral wool fibre coloured tuftedcarpet. The print head was operated at a frequency of 2 kHz to achievesubstantially uniform coloration of the individual fibres within thepile. The boundaries between different colours of the printed image wereclearly defined and the definition of the image was excellent. In analternative operation, the pint head was programmed not print an ink dotat the boundary between two colours so as to minimise the risk of colourbleeding between areas of different colours.

The calibration of a solenoid valve using software will now be describedwith respect to FIGS. 9 to 13.

FIG. 9 shows a schematic depiction of a solenoid valve 10 which issuitable for use with the method of the present invention. The valve 910comprises plunger 920, tube 930 and coils 940. The plunger 920 comprisesa ferromagnetic material (or any other magnetic material) and isreceived within the tube 930 so as to be able to move freely along theaxis of the tube. The plunger can be impelled, for example towards theopen end of the tube, by the application of a current to the coils 940,the current generating a magnetic field within the tube, which causes amagneto motive force to act upon the plunger. The timing and frequencyof the current pulses applied to the coils can be controlled by computer(not shown). The solenoid valve additionally comprises a returnmechanism (not shown), such as a spring, that acts to return the plungerto its initial position once the plunger has completed its full range oftravel.

In practice, a print head will comprise a matrix of such valves that arearranged in a square or rectangular arrangement. FIG. 10 shows twoexemplary valves 210 a, 210 b from such a print head matrix 220.Associated with each valve is valve control means 215 a, 215 b, each ofthe valve control means being in communication with a central computersystem 230. The operation of each valve is controlled by thetransmission of control pulses from the central computer system 230 toeach of the valve control means 215 a, 215 b. The valve control meansare responsive to the central computer system such that the centralcomputer system is able to vary the time that the valves are held openfor. This controlled variation of the valve enables ink drops of adesired size to be produced for depositing upon the substrate 250.

The print head can be calibrated upon manufacture and then at periodicintervals during its operation. The central computer system instructsthe print head to generate a predetermined matrix of drops. This testmatrix is deposited on a test substrate and the printed image can beexamined to determine the correlation of the printed image to theoriginal test matrix. If the ratio of the size of a printed pixel to thesize of the respective pixel of the original test matrix is outside athreshold value then the respective valve control means can beinstructed to change the time that the valve is to be opened for. If theprinted pixel is too small then the valve open time will be increased(either by the addition of more time or by multiplying the valve opentime by a suitable constant). Similarly, if the printed pixel is toolarge, then the valve open time will be decreased accordingly. Thethreshold that is used to determine whether a printed pixel is too smallor too large may be varied in accordance with the nature of the printsubstrate and/or the application that the print head is being used for.

As variations in printed pixel size will depend upon mechanicalvariations within the valve, it is possible that a valve may operatesatisfactorily for one size of pixel or within a given range of valveoperating rates. Therefore, the calibration may need to be repeatedacross the range of pixel sizes and valve rates that will be used by thevalve. The range of calibration factors that are required by each valvemay be stored in a look-up table, or it may be possible to determine oneor more equations such that the relevant calibration factor can becalculated given the desired valve operation rate and pixel size.

In an alternative embodiment, imaging means 240 may be additionallycoupled to the computer control system and aligned so as to view thearea of the substrate that the print head matrix prints upon. When atest matrix is printed upon the substrate, the image means is able toconvert that image to an electrical signal that can be transmitted tothe central computer system. The central computer system can, after anynecessary image processing (digitising, filtering, etc.), compare theprinted image with the original test matrix that is stored within thecentral computer system. The ratio of pixel sizes can be determined foreach pixel and calibration factors calculated for each valve asrequired. The central computer system can then communicate thecalibration factors to the valve control means associated with thevalves that require calibration.

The valve control means receives, interprets and executes signals thatare received from the central computer system. It will be readilyunderstood that the valve control means may be implemented such thateach valve has a dedicated control means or alternatively that a numberof valves may be controlled by a single control means.

In a preferred embodiment, the valve control means comprise a fieldprogrammable gate array (FPGA). FPGAs comprise memory and logic elementsthat can be configured by the user to provide a desired functionality.

In the preferred embodiment, the FPGA, and associated devices, is usedto control a linear array of 16 valves. Referring to FIG. 11, the valves610 a, 610 b, . . . , 610 p are controlled by valve control means thatcomprise FPGA 616, electrically erasable programmable ROM (EEPROM) 617,RAM 618, programmable ROM (PROM) 619 and input/outputs 622, 624, 626.The FPGA 616 is connected to each of the valves 610 a, 610 b, . . . ,610 p, EEPROM 617, RAM 618 & PROM 619. All three input/outputs 622, 624,626 interface with the FPGA. When the FPGA is powered up, it loads itsinternal configuration data from PROM 619 and then follows the sequencesthat have been loaded from the PROM. The EEPROM 617 stores a range ofdata comprising a look-up table comprising data associated with each ofthe valves, data specific to the valve control means and FPGA, statusinformation, etc. The FPGA will load this data from the EEPROM and theninitialise the RAM 618, by writing zero values into each memory locationin RAM. The FPGA will then wait to receive print data or other commandsfrom one of the inputs. Input/output 622 is connected to the computercontrol system and input/output 624 can be used to connect to a furthervalve control means (see below). Input 626 provides a series of pulsesthat are used in co-ordinating the printing process. When the array ofvalves is printing onto a substrate, the substrate is normally movedunderneath the valves. The series of pulses supplied to input 626 may begenerated from an encoder applied to a shaft in the apparatus that ismoving the substrate relative to the valves.

FIG. 12 shows a schematic depiction of a number of registers that areformed with the FPGA when the FPGA configuration data is loaded fromPROM 619. The first register 631 is used to write to and read from theEEPROM 617 and is also used when initialisation data is read from theEEPROM. Second register 632 receives print data from the computercontrol system, such as the alphanumeric characters or bitmaps to beprinted, or a signal to initiate a printing process. Second register 632also writes print data to the RAM and is used to initialise the RAMduring the start-up phase. The third register receives configurationdata from the computer control system such as data controlling the slantthat may be applied to the print head. Fourth register 634 receivesprint data from the RAM and passes it to the fifth register 635, whichuses the print data to operate the valves 610.

A desired print image (which may include alphanumerical characters) isentered into the computer control system and this image is thenconverted into raster data that is to be communicated with the valvecontrol means. The valves 610 may be operated for different periods oftime so as to provide the appearance of 16-level greyscale images. Thusthe print data can be supplied in the form of a raster comprising a 4bit word for each valve, with the value of the 4-bit word determiningthe greyscale that is to be generated by the valves. The print data isreceived by the second register and written into the RAM 618. The RAM islogically arranged in 16 rows, with each of the valves corresponding toa row. There are a plurality of columns, each of which corresponds to atime slot. Each raster scan also corresponds to a time slot and the timeslot is determined by the frequency at which the shaft encoder suppliespulses to the FPGA.

When print data is received at the FPGA the second register interpretsthe greyscale data for each valve, obtaining the time that each valvemust be opened for in order to generate the desired greyscale from alook-up table held in the first register. In theory, each valve shouldbe held open for the same period of time in order to generate the damegreyscale, but mechanical variations in each valve will lead to eachvalve having slightly different characteristics. Calibration factorsthat account for these differences are held in the look-up table. Thevalve times are then written into the RAM, using as many columns as arenecessary to store all of the rasters. A write pointer is set to thefirst column of the data. Each memory location holds the grey scalevalue for the associated valve and time slot.

When the next shaft encoder pulse is received the RAM column indicatedby the write pointer is read to see which of the 16 valves need to beoperated, i.e. which memory locations have non-zero entries. Once thememory locations have been read then all the memory locations in thecolumn are overwritten with zero.

The identity of these valves, along with the time for which the valvesare to be held open are then transmitted to the fourth register, whichmay perform further operations on the valve times in order to correctfor valve operation at high speed or a long time period betweensubsequent operations of the valve. The valve times are then passed tothe fifth register which calculates the number of shaft encoder pulsesthat are equivalent to the valve times. The valves are then opened for aperiod of time equal to that number of shaft encoder pulses.

As the valves 610 are electromechanical devices, their size provides alimitation to the print resolution that can be obtained. Typically, eachvalve may be provided at an offset of 4 mm from the adjacent valve(s).If a greater resolution (i.e. smaller pixel separation is required) thenthe matrix may be slanted so that the valves are closer together in oneaxis. The disadvantage of this is that if no correction is made to theprint rasters then the desired image will be printed out slanted.

Such a correction may advantageously be provided using the RAM toprovide a slant to the print raster data. Once the greyscale data hasbeen translated into valve open times, rather than writing the valvedata into a vertical column, the write data can be offset across anumber of columns. For example, if the desired slant angle is 45° thenthe valve open time for the first valve should be written into thecolumn indicated by the write pointer, the valve open time for thesecond valve should be written into the next column along from thecolumn indicated by the write pointer, and so on, such that the valveopen time is written into the RAM at the desired slant angle.

Typically the 16-level greyscale can be provided using valve open timesbetween approximately 80 .mu.s and 250 .mu.s. It has been foundadvantageous to initially open the valve by providing a first voltagefor a first period of time and to provide a second voltage, that islower than the first voltage, for a further period of time in order tohold the valve open. This reduces the possibility that the valve remainsopen for longer than is required to provide the desired greyscale,leading to decreased printing performance. It has been foundparticularly advantageous to apply a 36V pulse for approximately 80.mu.s and a second pulse of approximately 5V for the remainder of thetime that the valve remains open.

In a further preferred embodiment, the valve control means and valvesdescribed above with reference to FIG. 11 will be co-located upon asingle circuit board 650. A number of circuit boards can then beconnected in serial and physically located in a vertical array so thatthe valves can deposit a two-dimensional matrix on a print substrate. Insuch a case (see FIG. 14), one of the boards 650 a will be connected viaserial input/output 622 to the computer control system 230 and to thesecond board via serial input/output 624. The second board 650 b will beconnected to the first board via serial input/output 622 and to thethird board 650 c via serial input/output 624, and so on. The last boardin the serial chain can detect its position as its serial input/output624 will have no connection. On power up the last board in the serialchain assigns itself address 0 and transmits this address to thepreceding board, which then assigns itself address 1. This processcontinues, with the address value being incremented until each board hasan assigned address. The first board 650 a will then report its addressto the computer control system such that the system is aware of thenumber of connected boards. The system will prefix any communicationwith a board with the board's address. Preferably 16 boards areconnected together to provide a 16.times.16 printing matrix.

The FPGA used in the preferred embodiment was a Xilinx Spartan IIXC2S100 which was preferred as its configuration was determined by thedata loaded from the PROM in start up. Such an FPGA may be replaced by acheaper device in which the FPGA is hardwired, for example by blowingfuses to form logic elements, rather than configurable through software.

It will be understood that the above technique for calibrating asolenoid valve is suitable for use with any type of solenoid valve andin any application in which solenoid valves are used. However, thetechnique is of especial application to the compact high speed valves ofthe invention where the small size of the components makes manualadjustment of the position of pole pieces and other components difficultand inaccurate.

As stated above, the software and computer control can be used todecelerate the movement of the plunger at either or both extremes of itstravel so as to reduce spattering of the ink from the nozzle orifice dueto excessive slamming of the plunger against its seat.

According to another aspect of the present invention there is provided amethod of operating a solenoid valve, the method comprising the step ofenergising an electric coil to generate a magnetic field in order toreciprocally drive a plunger within a coil, wherein the magnetic fieldis controlled such that the speed of the plunger is decreased as theplunger approaches at least one of its extremes of movement. The controlof the magnetic field may be achieved in a number of ways.

In a preferred embodiment, the magnetic field may be controlled suchthat the speed of the plunger is decreased as the plunger approaches itsclosed position, in order to reduce the impact as the valve closes. Themagnetic field may be controlled such that the speed of the plunger isdecreased, the magnetic field resisting a force exerted on the plungerby a return means. Such a method of operating the valve is now describedwith reference to FIGS. 14 to 16.

FIG. 14 shows a schematic depiction of a solenoid valve 710 which issuitable for use with this method of operating the valve. The valve 710comprises plunger 720, tube 730 and coils 740. The plunger 720 comprisesa ferromagnetic material (or any other magnetic material) and isreceived within the tube 730 so as to be able to move freely along theaxis of the tube. The plunger can be impelled, for example towards theopen end of the tube, by the application of a current to the coils 740,the current generating a magnetic field within the tube, which causes amagneto motive force to act upon the plunger. The timing and frequencyof the current pulses applied to the coils can be controlled by computer(not shown). The solenoid valve additionally comprises a returnmechanism (not shown), such as a spring, that acts to return the plungerto its initial position once the plunger has completed its full range oftravel.

Conventionally, current is supplied to energise the coils as a simplesquare wave (or as a triangular wave having a steep gradient) in orderto provide a rapid acceleration of the plunger towards the closed end ofthe tube. Similarly, once the plunger has reached its maximum travelwithin the tube, the current is reduced quickly in order to reduce themagnetic force acting on the plunger quickly. This is advantageous asany magnetic force will oppose the force exerted upon the plunger by thereturn mechanism and thus the greater the magnetic force, the slower thereturn time of the plunger.

However, it has been established that in some high-speed applicationsfor solenoid valves, such as their use within ink jet printers, andnotably within ‘drop on demand’ ink jet printers, the increased rate atwhich magnetic forces are applied to and removed from the plunger arehaving a deleterious effect upon the operation of the valve.

FIG. 15 shows a schematic depiction of a solenoid valve 710 which isused within a drop on demand inkjet printer. The plunger 720 is, in itsclosed position, received upon an end of a nozzle bore 750 so as to sealthe nozzle. The tube 730 extends out at its open end to form a chamber760 which comprises an inlet 770 through which ink can be supplied.Energising the coils 740 through the application of an electricalcurrent to the coils impels the plunger along the tube, towards theclosed end of the tube. The movement of the plunger unseals the end ofthe nozzle bore, allowing ink to flow through the nozzle orifice. Oncethe magneto motive force (MMF) is removed from the plunger (through theremoval of the current from the coils 740) the return mechanism (notshown) returns the plunger to its closed position such that the plungeracts to seal the nozzle bore. Some form of seal or baffle may be appliedto the nozzle bore and/or the end face of the plunger in order toenhance the seal between the plunger and the nozzle bore so as to reducethe probability of ink entering the nozzle bore when the plunger is inits rest position.

It is desirable for the return mechanism to return the plunger to itsclosed position quickly to avoid the nozzle being left open for toolong: thus it is important to turn off the current pulse to the coils740 as soon as possible. The reciprocating motion of the plunger withinthe tube and the chamber is controlled so that a precisely controlleddrop of ink will be ejected from the nozzle orifice to be deposited upona substrate (not shown). When the valve is operated at high frequencies,typically from 2 to 4 kHz. Operation at such high speeds can causeproblems due to the method by which the coils are energised.

The energising of the coils causes the plunger to undergo a rapidacceleration until its motion is impeded by the end of the tube. Onlythe damping effect of the fluid within the tube and the force exerted bythe return mechanism opposes the motion of the plunger caused by theenergising of the coils.

The abrupt nature of the plunger's motion causes the formation ofsatellite droplets around the intended drops that are printed on thesubstrate. It is believed that the rapid acceleration of the plunger asit moves away from its rest position is responsible for the formation ofthese satellite droplets and that the problem is exacerbated due to thelimited fluid damping provided by the fluid within the tube. Furthermoreit has been observed that, if the force exerted upon the plunger by thereturn means is too great and if the magnetic force applied to theplunger is minimal as the plunger returns to its rest position, then theimpact of the plunger on the inlet to the nozzle bore can cause damageto the structure of the plunger or the nozzle (or to any sealing meansprovide on the nozzle or the plunger).

The possibility of producing such satellite droplets can be reduced byaltering the method by which the plunger is impelled. Rather than usinga square (or triangular) current pulse to energise the coils asdescribed above, the current is applied to the coils in a more gradualfashion. Similarly, if the manner in which the coils are de-energised iscontrolled appropriately, then the deceleration of the plunger will beless abrupt, which should serve to further reduce problems which arecaused by the impact of the plunger on the nozzle bore inlet. It isbelieved that similar problems may occur when solenoid valves areoperated at very high speeds in applications other than ink jetprinting.

The current may be applied as a generally triangular pulse (which may ormay not be symmetrical in the time domain), as a generally Gaussianpulse, a generally sinusoidal pulse or some other form of non-squarepulse that reduces the initial acceleration and final deceleration ofthe plunger. The exact nature of the solenoid valve and the rate atwhich it is being opened will determine whether or not the abruptacceleration and deceleration of the plunger has a deleterious effectupon the operation of the solenoid valve.

FIG. 16 a shows a graphical indication of a typical triangular wave thatis conventionally used to energise the coils. FIG. 16 b shows agraphical indication of a triangular wave that is used according to themethod of the present invention to energise the coils. It can be seenthat in the first part of the waveform shown in FIG. 16 b the gradientof the wave is less than for the waveform of FIG. 16 a. This ensuresthat the plunger is accelerated away from its rest position at a slowerinitial rate, reducing the possibility of forming satellite droplets. Itwill also be noticed that in the latter part of the waveform there is agreater current so that the magnetic force exerted upon the plunger actsto damp the motion of the plunger, moderating the effect of the returnmechanism. It will be understood that the exact waveform will bedependent upon, amongst other factors, the nature and structure of thesolenoid valve, the speed at which it is operated, the application inwhich it is used, etc., and that the waveform shown in FIG. 16 b ispurely exemplary.

Experimentation can be used to determine a suitable, or optimum,waveform or set of waveforms for use with a particular application. Onewaveform that was found to be of advantage is shown in FIG. 16 c. It hasbeen found advantageous to initially open the valve by providing a firstvoltage for a first period of time and to provide a second voltage, thatis lower than the first voltage, for a further period of time in orderto hold the valve open. This reduces the possibility that the valveremains open for longer than is required to provide the desiredgreyscale, leading to decreased printing performance. It has been foundparticularly advantageous to apply a 36V pulse for approximately 80.mu.s and a second pulse of approximately 5V for the remainder of thetime that the valve remains open.

During the high speed operation of solenoid valves in ink jet printing,the ink drops being deposited on a substrate can be monitored using aCCD (charge coupled device) camera coupled to a computer control systemto determine the number of problem satellite droplets that are beingformed and the frequency with which they are being formed. The collecteddata can be analysed by the computer, which can vary the current pulsesaccordingly to reduce the number of satellite droplets being formed. Thecomputer may select a current pulse from a range of pulses stored inmemory, along with an indication of the likelihood of a given currentpulse reducing the formation of satellite droplets.

In a preferred embodiment, the valve control means comprise a fieldprogrammable gate array (FPGA) using the circuitry shown in FIG. 12.FPGAs comprise memory and logic elements that can be configured by theuser to provide a desired functionality.

1. A method for applying an image forming composition to a pile fabricusing a drop on demand ink printer, characterized in that the printer isoperated at a drop generation frequency of at least 1 kHz.
 2. A methodas claimed in claim 2, characterized in that the pile fabric has a pilelength of at least 2 mms and the printer is operated at a pressure ofless than 3 bar, notably at from 1.5 to 2.5 bar.
 3. A method forapplying an image forming composition to a pile fabric using a drop ondemand ink jet printer, comprising the steps of: a. providing a drop ondemand ink jet printer, comprising:
 1. a reservoir;
 2. a nozzle orifice;and
 3. a valve for regulating the flow of ink from the reservoir to thenozzle orifice, the valve comprising: i. a plunger member journalled foraxial reciprocation between a rest and an operative position within atubular member supporting an electric coil under the influence of amagnetic field generated by that coil when an electric current passesthrough the coil; ii. bias means to bias the plunger towards its restposition when no current is applied to the coil, the distal end of theplunger extending into a valve head chamber having a outlet bore to anozzle orifice, the reciprocation of the plunger being adapted to openor close a fluid flow path from the valve head chamber through that boreto the nozzle orifice; and wherein, the plunger is made from anelectromagnetic material having a saturation flux density greater than1.2 Tesla, the bore leading from the valve head chamber to the nozzleorifice has a length to diameter ratio of 5:1 or less, and the nozzleorifice has a diameter of 80 micrometers or less; and b. operating theprinter at a drop generation frequency of at least 1 kHz
 4. A method forprinting an image on a pile fabric, using a drop on demand printer inwhich the solenoid valve mechanism for controlling the flow of fluid tothe nozzle orifice comprises a plunger member journalled for axialreciprocation between a rest and an operative position within anelectric coil under the influence of a magnetic field generated by thatcoil when an electric current passes through the coil, the distal end ofthe plunger extending into a valve head chamber having an outlet nozzlebore, the reciprocation of the plunger being adapted to open or close afluid flow path from the valve head chamber through that bore,characterized in that: a. the plunger is of a unitary construction andis made from an electromagnetically soft material having a saturationflux density greater than 1.4 Tesla, a coercivity of less than 0.25ampere per meter, and a relative magnetic permeability in excess of10,000; and b. the nozzle bore leading from the valve head chamber tothe nozzle orifice has a length to diameter ratio of less than 8:1.
 5. Amethod of operating a solenoid valve, the method comprising the step ofenergizing an electric coil to generate a magnetic field in order toreciprocally drive a plunger within a coil, wherein the magnetic fieldis controlled such that the speed of the plunger is decreased as theplunger approaches at least one of its extremes of movement.